1. Field of the Invention
The present invention relates to a communicator for field instruments which is connected to a transmission line which connects the field instruments to a host instrument in order to perform communication between the field instruments and the host instrument. The invention also relates to a method of supplying power to the communicator.
2. Description of the Related Art
Instruments known as field instruments have a great variety of sensors incorporated in them, and measure physical quantities, such as pressure, temperature, and flow rate in various plants. They transmit such physical measurements to a host instrument over a transmission line, after having converted the physical quantities into electric signals. The transmission of these electric signals has been standardized. The field instruments output analog current signals of 4-20 mA to the transmission line, and the host instrument receives the analog current signals. The analog signals are transmitted from the field instruments to the host instrument in a one-way communication.
Because of improvements in the technique of manufacturing semiconductor ICs, field instruments incorporating microprocessors have been developed and put into practical use in recent years. The field instrument performs two-way communication in digital signals, rather than one-way communication in analog signals as over the above-mentioned transmission line, and is capable of performing processes such as range setting and self-diagnosis of the field instrument even from a remote place. The field instrument also communicates with a communicator exclusively in digital signals, this communicator being connected to any place along the transmission line. A device of this type is disclosed, for example, in Japanese Patent Laid-Open No. 59-201535.
In the conventional example mentioned above, as a method of transmitting signals over the transmission line, digital signals are carried on analog signals for simultaneous communication. In addition to this method, there is a method in which analog signals are switched over to digital signals for communication. There is also a method in which communication is performed solely in digital signals.
In these conventional examples, the communicator has a built-in power supply such as a battery, and is constructed so as to operate all the built-in circuits on the electric power fed from the built-in power supply. For this reason, it is required to carry out maintenance, such as replacing or charging the built-in battery, after the built-in battery has been used for a predetermined period.
The above-described conventional art, however, does not take into consideration where the communicator is continuously used for a long period of time for trouble-shooting the field instrument or the like. In other words, there is a problem in that since the service time of the power supply incorporated in the communicator is limited, it is impossible to continuously monitor values, such as output values and internal status of the field instrument for prolonged periods.
Furthermore, the communicator is not always utilized in an instrument room, but may also be connected to any place along the transmission line for outdoor use. In such a case, when the capacity of the built-in power supply runs out during its service, the built-in power supply must be replaced or charged. This leads to a problem in that maintenance, such as replacing or charging the built-in power supply, becomes troublesome.